1. Field of the Invention
This invention relates to the shaping and cooling of glass sheets and particularly in the high speed production of bent glass sheets that are toughened by air quenching, and most particularly, for shaping and heat treating relatively thin glass sheets. The present invention is especially concerned with the heat treatment and, most particularly, the rapid cooling of glass sheets that are supported in an essentially horizontal disposition during said treatment, although alternate embodiments can be used for treating glass sheets supported in vertical or oblique dispositions.
Shaped glass sheets are widely used as side windows in vehicles such as automobiles or the like and, to be suitable for such application, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass side windows are installed. It is also important that the side windows meet stringent optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area. During fabrication, glass sheets intended for use as shaped windows in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increasing the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of glass sheets to breakage, tempered glass sheets, if fractured, break into relatively small, relatively smoothly surfaced fragments that are less injurious than the relatively large, jagged fragments that result from the more frequent breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. During such treatment, a glass sheet is conveyed along a path (usually horizontal, although vertical paths and paths having both horizontal and vertical components are also known) that extends through a tunnel-type furnace where the glass sheet is one of a series of sheets that are heated to the deformation temperature of glass and into a shaping station where each glass sheet in turn is shaped to a desired configuration. The shaped glass sheet is then moved along a horizontal or vertical path between spaced, opposed plenum chambers that dispense tempering medium, usually in the form of air blasts imparted through a first array of nozzles that face the concave surface of the shaped glass sheet and a second array of nozzles that face the convex surface of the shaped glass sheet as the latter is conveyed along said path between said spaced, opposed arrays of nozzles. The nozzle arrays are found in a cooling station.
With few exceptions, in the prior art, the first array of nozzles was arranged and constructed as identical as possible to the second array of nozzles so that each nozzle of the first array faced a corresponding nozzle of the second array and the type of nozzle (pipe or slot or "rosette" module) in any portion of the first array of nozzles was opposed by a corresponding nozzle of the same type in the second array of nozzles for either the entire cooling station or at least almost the entire length of said cooling station.
When prior art apparatus cools a shaped glass sheet that was not heated sufficiently for tempering, temporary tension stresses are established in the glass that sometimes cause breakage. The lower array of nozzles used to treat glass sheets supported in a horizontal disposition is mounted on a pivoted structure to help remove glass particles rapidly by pivoting the lower array downwardly when breakage occurs and returning the lower array to a horizontal orientation when the glass fragments are removed by sliding. Preferably, resumption of the horizontal orientation takes place before a succeeding glass sheet reaches the location of the breakage of a previous glass sheet in the cooling station.
In the past, the lower nozzles that pivoted were in the form of apertured pipes having apertures spaced along and extending through the walls thereof. Such apertured pipes did not deliver tempering medium as rapidly as pipes or slots having elongated passages to guide the movement of said tempering medium against glass sheet surfaces in directions oblique or normal to the major glass sheet surfaces, thus providing increased cooling effect on the glass surface and a higher degree of temper for a given rate of flow of tempering medium. Where the apertured pipes of both the upper and the lower array are apertured through the wall thickness of the pipes, both the upper and lower glass surfaces develop a level of temper that is less than desired when the glass sheets are quenched at a rate of flow that provides sufficient cooling to develop a barely acceptable temper when blasted at said rate of flow through elongated pipe-type or slot-type nozzles.
2. The Prior Art
U.S. Pat. No. 3,372,016 to Donald D. Rahrig et al. discloses glass sheet tempering apparatus that has opposite arrays of nozzles provided with slotted openings in a cooling station thereof. The upper array of slotted openings oppose the lower array of slotted openings.
U.S. Pat. No. 3,846,106 to Seymour provides glass sheet tempering apparatus in which glass sheets are conveyed between an upper and a lower array of apertured pipes. The apertures extend through the wall of the pipes in a radial direction. Tempering medium (air) is supplied under pressure through the pipes for release through the apertures against the opposite surfaces of a shaped glass sheet. The lower array of pipes is pivoted to facilitate removal of glass fragments if breakage occurs.
U.S. Pat. No. 3,881,906 to George F. Ritter et al. discloses glass sheet tempering apparatus comprising upper and lower arrays of nozzles for delivering tempering medium against the upper and lower major surfaces of glass sheets moving therebetween through a cooling station. The upper and lower arrays are essentially identical to one another except for the fact that the nozzles have free ends that terminate along convexly and concavely curved surfaces that are complementary to each other and match the curvature of the glass sheets transverse to the path of glass sheet travel through the cooling station. The upstream nozzles are directed obliquely away from the furnace exit and are in the form of pipes. Additional pipe type nozzles are provided that impart tempering medium in a direction normal to the horizontal plane of the path of glass sheet travel. Further downstream, both upper and lower arrays of nozzles comprise tubes that extend through thick walls of opposing air chambers. Such a construction is used to provide flat, trap-free surfaces facing the glass sheets, which facilitates removal of broken glass from the blast heads in a potentially high breakage area.
French Pat. No. 2,024,397 discloses glass sheet tempering apparatus that has an upper array of slit type nozzles opposing spaced rows of so-called "rosette" modules that provide both a source of cooling medium and a support bed for flat glass sheets conveyed through the cooling area of a glass sheet tempering apparatus.